Amorphous metallic alloys (i.e., metallic glasses) represent a relatively young class of materials, having been first reported around 1960 when classic rapid-quenching experiments were performed on Au—Si alloys. Since that time, there has been progress in exploring alloys compositions for glass formers, seeking elemental combinations with ever-lower critical cooling rates, which may still retain an amorphous structure. Due to the absence of long-range order, metallic glasses may exhibit relatively unique properties, such as high strength, high hardness, large elastic limit, good soft magnetic properties and high corrosion resistance. However, owing to strain softening and/or thermal softening, plastic deformation of metallic glasses may be highly localized into shear bands, which may result in a limited plastic strain (e.g., less than 2%) and failure at room temperature.
Different approaches have been applied to enhance ductility of metallic glasses including: introducing heterogeneities such as micrometer-sized crystallites, or a distribution of porosities, forming nanometer-sized crystallites, glassy phase separation, or by introducing free volume in amorphous structure. The heterogeneous structure of these composites may act as an initiation site for the formation of shear bands and/or a barrier to the rapid propagation of shear bands, which may result in a relative enhancement of global plasticity, but may sometimes decrease strength. It should be noted, that while some metallic glasses may exhibit relatively enhanced plasticity during compression tests (12-15%), their response in unconstrained loading may be much different and the tensile elongation may not exceed 2%.
Relatively recent results on improvement of tensile ductility of metallic glasses indicated that 13% tensile elongation may be achieved in zirconium based alloys with large dendrites (20-50 μm in size) embedded in a glassy matrix. It should be noted that this material is primarily crystalline exhibiting 50% or greater crystallinity by volume and might be considered a microcrystalline alloy with a residual amorphous phase along dendrite boundaries. Furthermore, the maximum strength of these alloys may be relatively low at 1.5 GPa and ductility may only be achieved after the yield point is exceeded, resulting in strain softening which may not be considered industrially usable. Thus, while metallic glasses are known to exhibit somewhat favorable characteristics including relatively high strength and high elastic limit, their ability to deform in tension may be limited, which may limit the industrial utilization of this class of materials.